1. Field of the Invention
This invention relates to stacking cases of various size, shape, and weight on a pallet for transport. More particularly the invention pertains to a computer-controlled method for selecting cases presented in random order and using an industrial robot to produce a stable stack of cases on the pallet.
2. Description of the Prior Art
In the transportation and distribution of goods, cases, boxes or cartons containing goods are stacked on a portable platform or pallet that is readily moved by a vehicle from a palletizing area. The cases frequently vary in size, shape, and weight and usually arrive in a random order in the area where they are placed on the pallets. This usually results in a random stacking of the cases until the pallet is either completely full or cannot be stacked higher. The pallets are usually then enveloped by shrink-wrap plastic material to prevent inadvertent displacement of the cases during shipping.
Attempts have been made to load unequal size and weight cases by automated palletizing systems. In a first system, an operator having knowledge of the size and weight of all of the cases to be loaded indicates to a computer, with the aid of a graphic terminal, the location where each case should be placed on the pallet. The computer determines the optimum loading sequence, another device puts the cases in that optimal order, and the cases are loaded on the pallet by a robot arm controlled by the computer. U.S. Pat. Nos. 4,641,271; 4,692,876 and 4,746,255 describe palletizing systems that require prior knowledge of all of the cases to be palletized. The processes they describe are slow, and the palletized stack is not optimized for stability.
It is difficult to load cases manually onto a pallet so that the stacked cases are stable when different size and weight cases are presented in random order for palletizing. To overcome this difficulty, industrial robots are used to stack cases onto pallets when the cases are of the same size and weight, or the cases arrive in a predetermined order at the palletizing area. Expert systems are now available, which automatically determine the configuration for different cases having various sizes and weights, but without operator assistance, provided the system has prior knowledge of the size and weight of every casein the entire pallet load.
U.S. Pat. No. 5,175,692 describes a method and computer-assisted apparatus for automatically palletizing parcels of various sizes, shapes and contents, which arrive in random order at a pickup point to be transferred onto a pallet. The system includes an expert computer program, which uses attribute factors of the parcels, applies predetermined rules to assign a space for the parcel on the pallet, and communicates coordinates of the assigned space to a material handling robot.
U.S. Pat. No. 5,501,571 describes an automatic palletizer system that loads a random set of different size and weight cases onto a pallet. As the cases pass along a conveyor, their weight, dimensions, and contents are determined, a processor continuously maps the optimal pallet configuration for the remaining cases as the pallet is loaded, and reshuffles the sequence of a limited number of cases to improve the final pallet configuration.
U.S. Pat. No. 5,908,283 describes a method and apparatus for palletizing rectangular packages received in random size and weight. A corner-based modeling system is used to assist in evaluating possible placements of packages accumulated on a line conveyor, and a placement evaluation process is used to select a best package placement based on heuristic analysis. Statistically-based measurement and comparison is used to assist in the evaluation process.
In methods that employ multiple rules to determine a location for a case on a pallet, a choice among the results of applying the various rules is made with reference to a best score or single rule approach, which may not produce an optimal choice. Often the methods require large computing capacity to process a large number of pallets in a limited time, or to control several robots in real time. Current methods often produce a stacked pallet that has poor stability because of emphasis placed on maximizing pallet volume. This emphasis tends to create towers of similar shaped cases. Because current methods place cases in inaccessible cavities between previously stacked cases, complicated tooling and controls are required to access the cavities.
Because of these deficiencies, a need remains for an automated palletizing system that can continually determine an optimal case configuration while loading the pallet as cases of mixed size, shape and weight are presented randomly to a buffer for stacking on the pallet.